feat: Add advancedStepInput feature flag for multi-select and drag-to-paint

Adds a combined feature flag to disable both multi-select (Ctrl+Click,
Shift+Click to extend) and drag-to-paint under a single toggle.

When disabled (VITE_FEATURE_ADVANCED_STEP_INPUT=false), the sequencer
reverts to classic TR-808 style interaction:
- Single click still toggles steps
- Drag only affects first clicked step (no paint mode)
- Ctrl+Click does nothing
- Shift+Click always opens p-lock menu
- Selection badge never appears

Default: enabled (preserves current behavior)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: adewale

app/src/components/StepSequencer.tsx

@@ -578,8 +578,8 @@ export function StepSequencer() {
         />
       </div>
 
-      {/* Phase 31F: Selection indicator badge */}
-      {selectionCount > 0 && (
+      {/* Phase 31F: Selection indicator badge (only when advanced input enabled) */}
+      {features.advancedStepInput && selectionCount > 0 && (
         <div className="selection-badge" title={`${selectionCount} step${selectionCount > 1 ? 's' : ''} selected • ESC to clear • Delete to remove`}>
           <span className="selection-count">{selectionCount}</span>
           <span className="selection-label">selected</span>
@@ -642,10 +642,10 @@ export function StepSequencer() {
                   onEuclideanFill={(hits) => handleEuclideanFill(track.id, hits)}
                   onSetName={(name) => handleSetName(track.id, name)}
                   onSetTrackSwing={(swing) => handleSetTrackSwing(track.id, swing)}
-                  selectedSteps={selectedSteps}
-                  selectionAnchor={selectionAnchor}
-                  hasSelection={selectionCount > 0}
-                  onSelectStep={(step, mode) => handleSelectStep(track.id, step, mode)}
+                  selectedSteps={features.advancedStepInput ? selectedSteps : undefined}
+                  selectionAnchor={features.advancedStepInput ? selectionAnchor : undefined}
+                  hasSelection={features.advancedStepInput && selectionCount > 0}
+                  onSelectStep={features.advancedStepInput ? (step, mode) => handleSelectStep(track.id, step, mode) : undefined}
                   loopRegion={state.loopRegion}
                   isDragTarget={isDragTarget}
                   isDragging={isDragging}

app/src/components/TrackRow.tsx

@@ -13,6 +13,7 @@ import { useRemoteChanges } from '../context/RemoteChangeContext';
 import { getInstrumentCategory, getInstrumentName, TONE_SYNTH_CATEGORIES, SAMPLED_CATEGORIES } from './sample-constants';
 import { getTransposedRoot, type NoteName } from '../music/music-theory';
 import { isInRange, isInOptimalRange } from '../audio/instrument-ranges';
+import { features } from '../config/features';
 import './TrackRow.css';
 import './ChromaticGrid.css';
 import './PianoRoll.css';
@@ -342,13 +343,22 @@ export const TrackRow = React.memo(function TrackRow({
   const handlePaintStart = useCallback((stepIndex: number) => {
     const wasActive = track.steps[stepIndex];
     const newState = !wasActive;
-    setPaintMode(newState ? 'on' : 'off');
+
+    // Always toggle the clicked step
     onToggleStep(stepIndex);
+
+    // Only enable drag continuation if advanced input is on
+    if (features.advancedStepInput) {
+      setPaintMode(newState ? 'on' : 'off');
+    }
   }, [track.steps, onToggleStep]);
 
   // Continue painting: apply paint mode to entered step
   // Uses paintModeRef to avoid stale closure issues with paint mode
   const handlePaintEnter = useCallback((stepIndex: number) => {
+    // No drag painting when advanced input is disabled
+    if (!features.advancedStepInput) return;
+
     const currentPaintMode = paintModeRef.current;
     if (currentPaintMode === null) return;
     const isActive = track.steps[stepIndex];

app/src/config/features.ts

@@ -66,6 +66,16 @@ export const features = {
    * Default: true (stable feature)
    */
   multiplayer: parseEnvBool(import.meta.env.VITE_FEATURE_MULTIPLAYER, true),
+
+  /**
+   * Advanced Step Input - Multi-select and drag-to-paint
+   * Default: true (stable feature)
+   * When disabled, reverts to classic TR-808 style: click-to-toggle only
+   * - Ctrl+Click: Does nothing (instead of toggle selection)
+   * - Shift+Click: Always opens p-lock menu (instead of extend selection)
+   * - Drag: Only toggles first step (instead of paint mode)
+   */
+  advancedStepInput: parseEnvBool(import.meta.env.VITE_FEATURE_ADVANCED_STEP_INPUT, true),
 } as const;
 
 /**